The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast that includes a circuit for preventing start up of the inverter when one or more of the ballast output wires is shorted to earth ground.
A number of existing electronic ballasts have non-isolated outputs. Such ballasts typically include circuitry for protecting the ballast inverter from damage in the event of lamp fault conditions such as lamp removal or lamp failure.
Occasionally, the output wiring of a ballast becomes shorted to earth ground in the lighting fixture. Such a condition can arise, for example, due to the wires becoming loose or pinched. For ballasts with non-isolated outputs, if the inverter begins to operate while an earth ground short is present at one or more of the output wires, a very large low frequency (e.g., 60 hertz) current will flow through the inverter transistors and cause them to fail.
Thus, a need exists for a ballast with a protection circuit that prevents the inverter from starting when an output ground-fault condition is present. A ballast with such a protection circuit would represent a significant advance over the prior art.